The present invention relates to a method and apparatus for reproducing information in which information is recorded and reproduced at a high speed and high density using information reproduction means which uses magnetic force or a laser beam, and, more particularly, to a method and apparatus for optically reproducing information with a laser beam or the like.
In an information recording/reproduction system using an optical or magnetic disk as a recording medium, there are a mark-position recording/reproduction scheme and a mark-edge recording/reproduction scheme.
In recent years, attention is being focused on a mark-edge recording scheme in which information is provided for the leading and trailing edges of a mark as a technology enabling high density recording.
To reproduce the mark-edge recorded information, there have been known a second order differential zero-cross detection type in which a read signal is second order differentiated for detecting a zero-cross point corresponding to an edge position, and a DC slice detection type in which a read signal is binary coded by slicing it with a reference level.
The edge reproduction scheme of such second order differential zero-cross detection type has an advantage that an apparatus can be easily implemented with a simple circuit configuration since its characteristics are not affected even if there is a sudden direct current (DC) variation due to defects or immediately after erasure of record, but it is very sensitive to a reproduced S/N ratio and distortion of waveform.
That is, the edge reproduction scheme of the second order differential zero-cross detection type has various problems. Noises (medium noise, LD noise, circuit noise or the like) causing problems in reading may be enhanced by the differential, and an inflection point of a second order differential signal does not correspond to an edge position of a record mark due to waveform distortion. These factors deteriorate an information signal detection accuracy, disabling to reduce the reproduction error rate.
Then, as an example of means for solving such disadvantage, Japanese Patent Publication (Unexamined) No. A-4-232659 proposes to use amplitude limitation means for the edge reproduction scheme of the second order differential zero-cross detection type.
FIG. 5 is an example of arrangements for conventional information recording/reproduction apparatus, while FIG. 6 illustrates the operation of information recording/reproduction apparatus of FIG. 5.
Information recorded on an information recording medium is extracted by an optical head 1, and amplified by an amplifier circuit 2.
The amplified read signal (a) is corrected for resolution by a linear phase type waveform equalizer circuit 3 to obtain a signal (c) limited for amplitude with an amplitude limiting circuit 5. The amplitude limited signal (c) is differentiated by a differential circuit 7 to obtain a first order differential signal (d), and compared for level with positive and negative slice levels +V.sub.T and -V.sub.T by a change direction extraction circuit 9 to obtain binary signals (f) and (g) for a direction of signal change. In addition, the first order differential signal (d) is differentiated by a differential circuit 8 to obtain a second order differential signal (e), and an inflection point signal (h) of the read signal is extracted by a zero-cross comparator circuit 10. An edge of the read data signal is extracted and binary coded from a timing relationship between the binary coded signals (f), (g) and the inflection point signal (h).
In such prior art, it has been believed that the reproduction error rate of read data information can be reduced by improving the reproduction S/N ratio or eliminating waveform distortion with the amplitude limiting means even in the edge reproduction scheme in the second order differential zero-cross detection type.
However, when the amplitude limiting means is applied to the edge reproduction scheme in the second order differential zero-cross detection type, it is only to improve the reproduction S/N ratio, which is a disadvantage shortcomings of this scheme, narrowing the characteristic margin against sudden direct current variation, which margin is originally an advantage of this scheme.
Then, there is a problem that reproduction error tends to occur (although the reproduction S/N ratio is improved) because up to an original edge extraction point is eliminated and lost by the amplitude limiting means was the variation of direct current component becomes significant as in the case immediately after erasure of a record or a defect in the recording medium.